The present invention is directed to optical components for use in fiber optic networks and particularly to devices known as optical circulators and more particularly to polarization maintaining fiber optic circulators.
By directing signal flow in the proper direction, optical circulators can reduce system cost and complexity in optical equipment used in fiber optic networks. In complex optical networks, passive optical components are essential elements for sorting and delivering signals to their proper destination. To accomplish this control, the optical-signal flow through the sequential ports of a circulator is guided in a fashion comparable to that of vehicles entering and leaving a traffic circle. A circulator transmits an incoming signal entering Port 1 to Port 2 while transmitting another incoming signal from Port 2 to Port 3, and another from Port 3 to Port 4 etc. The number of ports can be increased arbitrarily, and it is possible to have fully circulating devices (also called an closed loop circulator), in which light entering the last port exits the first port, and quasi-circulating devices (also called an open loop circulator), wherein the light from the last port does not return to the first port, this quasi-circulator is the most common type. The performance advantages of optical circulators make them indispensable for routing bidirectional optical traffic. Firstly, optical circulators are low-loss devices, unlike splitters that incrementally add 3-dB losses for each device used. Secondly, optical circulators have high adjacent port isolation and eliminate the need for external isolators.
As fiber optic communication systems increase in complexity and functionality, the demand for increased capacity and efficient (low loss) signal routing management increases. For example, in duplex (bidirectional) transmission, the conventional use of fused fiber 3 dB couplers costs the system more than 6 dB in loss. The use of optical circulators in such cases saves about 5 dB""s due to the ability of circulators to route the signal in its entirety in the desired direction. Optical circulators are also important and enabling components in ADD/DROP applications. Optical circulators are forecast to play a significant role in duplex transmission, optical time domain reflectometry (OTDR) measurement systems, wavelength division/multiplexing (WDM) transmission systems and Erbium (Er) doped fiber amplifiers (EDFA).
The present invention provides polarization maintaining circulators of a number of embodiments. The first embodiment uses birefringent wedges and Faraday rotators and is an inline design, meaning the fibers are all inline with each other. This design is described in a 3 port version, however it can be extended to 4 or more ports. Inline designs are generally more compact, less complex and reduce alignment problems as compared to non-inline designs. The second embodiment makes use of a polarizing beam splitting cube and Faraday rotators which results in a design with the fibers being at either 90xc2x0 or 180xc2x0 with respect to each other, all in the same plane. This second embodiment is limited to a maximum of 4 ports but has the advantage of being a closed design, meaning that light launched from a port will eventually return to that port, for example light launched form port 1 will follow the following sequence: 1sxe2x86x922sxe2x86x923ssxe2x86x924sxe2x86x921s, for the fast axis the sequence is slightly different: 1 fxe2x86x924fxe2x86x923fxe2x86x922fxe2x86x921f. Note that the axes of polarization maintaining optical fibers are referred to as xe2x80x9cslowxe2x80x9d (or major) and xe2x80x9cfastxe2x80x9d (which relates to the relative propagation velocities).
A third embodiment makes use of beam splitter cubes and Faraday rotators, like the second. However the difference is now that instead of having 3 or 4 separate fibers, two fibers are combined into one holder, making this an inline device. This is accomplished by placing a reflector on one or both of the sides of the polarizing beam splitting cube (depending on a 3 or 4 port design). The four port design of this embodiment is of the closed variety, meaning that light launched into the slow axis of port 1 will follow the following route: 1sxe2x86x922sxe2x86x923sxe2x86x924sxe2x86x921s, light launched into the fast axis of port 1 will follow: 1fxe2x86x924fxe2x86x923fxe2x86x922fxe2x86x921f, which is similar to that of the previous design.